optimize direct timeline (#7614)

* optimize direct timeline

* fix typo in class name

* change filter condition for direct timeline

* fix codestyle issue

* revoke index_accounts_not_silenced because direct timeline does not use it.

* revoke index_accounts_not_silenced because direct timeline does not use it.

* fix rspec test condition.

* fix rspec test condition.

* fix rspec test condition.

* revoke adding column and partial index

* (direct timeline) move merging logic to model

* fix pagination parameter

* add method arguments that switches return array of status or cache_ids

* fix order by

* returns ActiveRecord.Relation in default behavor

* fix codestyle issue

app/controllers/api/v1/timelines/direct_controller.rb

@@ -23,15 +23,18 @@ class Api::V1::Timelines::DirectController < Api::BaseController
   end
 
   def direct_statuses
-    direct_timeline_statuses.paginate_by_max_id(
-      limit_param(DEFAULT_STATUSES_LIMIT),
-      params[:max_id],
-      params[:since_id]
-    )
+    direct_timeline_statuses
   end
 
   def direct_timeline_statuses
-    Status.as_direct_timeline(current_account)
+    # this query requires built in pagination.
+    Status.as_direct_timeline(
+      current_account,
+      limit_param(DEFAULT_STATUSES_LIMIT),
+      params[:max_id],
+      params[:since_id],
+      true # returns array of cache_ids object
+    )
   end
 
   def insert_pagination_headers